Because if Talib continues to battle injuries, or turns back into an ####### now that he's made his money, and you want to cut him, you can't do so without accelerating all that guaranteed money onto your cap. You were saying earlier that teams lose because the players that they paid to perform didn't play well. Well, in order for that to be true, they need to get paid first. In virtually every instance you're describing, the team expected the player to contribute a value deserving of the contract they're giving them. Not many teams sign players to big money not expecting it to work out.
I don't think many Jets fans care about the actual cash Decker would be taking in. It's if that extra $3MM (or $5MM, if they don't think he's truly worth more than a guy making $4MM) could be better deployed somewhere else on the roster, and it's a consideration made with every player in every league with a salary cap.
Regarding the bolded - That's not what I meant/you misunderstood. You need to take it at face value...I'm simply talking about talent. As an easy example, Talib is talented but Saffold is not.
The risk you take with the "what if's" are really not that different whether you're talking about character or possibility of injury. They both could potentially strike at any time.
So the question you have to ask is really one of talent.
Now, regarding the Decker/Jets example... this is a matter of supply, demand, and need. I don't think Decker is the best WR, but he's probably the best WR available and it happens to be at a position of (arguably) biggest need for the Jets. Talking about where $5m could be better spent somewhere else doesn't seem to make much sense when a) it's really not that much money, relatively speaking and b) what does "better spent" mean when you're talking about improving the worst area on your roster?